1. Technical Field
The present invention generally to wireless telecommunications and more particularly to a method for optimizing hard handoffs between Mobile Switching Centers in a Code Division Multiple Access (CDMA) network.
2. Description of the Related Art
Handoff is the act of transferring control of a mobile station from one base station (BTS) to another. In CDMA systems, handoff determinations are typically made based on signal strength measurements by mobile stations of pilot signals transmitted by respective base stations. If the measured pilot signal strength of a present base station (presently communicating with the mobile station) falls below a threshold, the mobile station transmits a Pilot Strength Measurement Message (PSMM). The traffic channel which received the PSMM forwards it to a transceiver and selector bank (TSB) of a base station controller (BSC). The base station controller determines what type of handoff should be performed.
CDMA handoffs are classified into three types. The first type is known as a soft handoff, which is a call state in which two or more base stations support a mobile station. A soft handoff usually occurs when a mobile station travels from one cell to another cell serviced by the same BSC, where the base station of the second cell uses the same frequency assignment as the first. The second type is known as a xe2x80x9csofterxe2x80x9d handoff, which is performed when a mobile station moves from one angular sector to another angular sector in the same BTS service area.
The third type of handoff, hard handoff, is an abrupt handoff in which the mobile station is not controlled simultaneously by two or more base stations. Unlike soft or softer handoffs, the call link connected to the mobile station is not always maintained during hard handoff; instead, it is cut off and re-established in a very short time frame. Hard handoff can occur either between mobile switching centers (MSCs), between frequency assignments, or between frame offsets. Hard handoffs are usually performed between same-protocol systems, but can also be performed between systems of different protocols, such as from a CDMA system to an analog Advanced Mobile Phone System (AMPS). Hard handoffs from CDMA to CDMA systems are categorized into: handoffs between base stations (or MSCs) of different manufacturers; handoffs between channels with different frame offsets; and handoffs between different frequency assignments. During an inter-MSC hard handoff, when a mobile station moves to a cell serviced by a neighboring MSC, the MSC is switched and the vocoder is reassigned.
Handoff parameters used to evaluate handoff are defined as follows:
T_ADDxe2x80x94a threshold value for a pilot-to-interference ratio (PIR). When the PIR of a pilot from a particular base station is measured by a mobile station to be higher than T_ADD, then the mobile transmits a message to the communication network to report the detection of that pilot. The base station that transmitted the pilot will then be added to a set of base stations that are candidates for accepting a handoff. The actual value of the threshold T_ADD is a parameter that the mobile obtains from the overhead messages broadcast by the base stations. If T_ADD is decreased, the handoff coverage area increases, the capacity decreases and the quality improves through diversity gain, since more traffic channels are available for soft handoff.
T_DROPxe2x80x94a threshold value for a pilot-to-interference ratio which will result in dropping a base station from a list of possible candidates for accepting a handoff. If the value of T_DROP is increased, the handoff coverage area decreases, capacity increases and quality is diminished.
T_COMPxe2x80x94a threshold parameter determinative of where a hard handoff can occur with respect to a target cell. If T_COMP is reduced, hard handoff occurs a farther distance away from the target cell, resulting in a higher probability of failure and an increase in the hard handoff ping-pong phenomenon (and vice versa if T_COMP is increased).
Guard timerxe2x80x94a guard timer which is similar to the T_TDROP parameter (discussed below) is managed by the Call Control Processor (CCP) or the Call Control Block (CCOX) to alleviate the hard handoff ping pong phenomenon. Hard handoff is not performed until the guard timer period has elapsed.
Guard levelxe2x80x94a threshold signal add strength level. Though the guard timer has already started timing, if the strength (Ec/Io) of the pilot received by the mobile station is xe2x88x9212 dB or below, a hard handoff is performed before the guard timer period is complete.
T_TDROPxe2x80x94a timing interval during which hard handoff is performed. FIG. 1 is a table showing a mobile station""s travel distance according to T_TDROP values for the case of a BTS servicing an urban area. As shown, when the mobile station travels 50 m during a four second interval (corresponding to T_TDROP=3), about 10% of the BTS coverage area of a typical 500 m long coverage area is traversed. Therefore, in an urban area where a number of active sets exist and the RF environment rapidly changes during short distances, the proper T_TDROP value for fast adaptation will be four seconds or less.
Tx_ATTENxe2x80x94the attenuation value of a base station transmit attenuator controlling base station transmit power. The final output (or power) variable of the BTS per traffic channel is the Tx_ATTEN value of the transceiver unit associated with that traffic channel. The output of the BTS can be changed as much as the Tx_ATTEN value, which is set when the input level to the transceiver is constant. The shadow area, the handoff area, and the link balance may be adjusted by varying TX_ATTEN. These three values are adjusted by remote control of the base station its manager (BSM) or by using a map from the BTS.
Antenna direction (azimuth) adjustmentxe2x80x94adjusting the antenna direction in azimuth changes the coverage area. When adjusting the antenna direction, the following conditions must be considered: 1) if the antenna is located in front of a building and the beam points to the building, the progress of the radio wave will be hindered; 2) reflections off bodies of water, e.g., rivers or lakes, may produce higher interference levels in desolate areas; 3) the antenna beam is affected by a lossy propagation medium, such as a park with many trees; 4) an area with low signal level and heavy communication traffic may be adjacent another area with less traffic and higher signal level relative to the amount of traffic.
Antenna tilt (elevation)xe2x80x94when adjusting the antenna beam direction in elevation, the following conditions must be considered: 1) when the antenna is located atop a tall building, the main lobe of the antenna beam may not point to the ground area proximate the lower part of the building, resulting in low signal levels thereat; 2) when the antenna has no tilt in a hilly area, it produces an interference signal to other base stations.
Conventionally, to achieve a specified signal level in a designated coverage area and/or to adequately transmit signals into a shadow area, the BTS transceiver power is first adjusted. If such adjustment fails to produce acceptable coverage in the coverage area or to resolve the shadow area problem, the antenna beam may be adjusted in azimuth or elevation. It is noted that a shadow area can be determined by a low Received Signal Strength Indicator (RSSI) measurement or by a low Ec/Io (signal to interference ratio) measurement in a particular area.
Techniques for improving call service based on signal strength measurements or handoff statistics have been described. For instance, U.S. Pat. No. 5,737,705 describes a method for improved frequency assignment based on handoff statistics taken to and away from a base station. U.S. Pat. No. 5,822,686 relates to signal strength measurements taken on varying radio channels to provide information to the system for channel allocation and handoff decisions.
At present, the soft and softer handoff techniques have been proven to exhibit superior call quality to hard handoff methods in terms of call failure rates and the like. Thus, there is a need for a way to improve the reliability of hard handoffs, particularly for those occurring between mobile switching centers (inter-MSC handoffs).
It is an object of the present invention to provide an optimization method for inter-MSC hard handoffs so as to enhance the total network quality.
In one aspect of the present invention, there is provided a method for optimizing hard handoffs in a Code Division Multiple Access (CDMA) network, e.g., between Mobile Switching Centers. In the method, preliminary conditions for a hard handoff test are first set up. Idle handoffs are then performed in a hard handoff area, log data for the idle handoffs is analyzed and adjustments to communication equipment are made accordingly. Next, hard handoff tests are conducted in a traffic state in the hard handoff area. Log data of the hard handoff tests are analyzed and further adjustments to communication equipment are made if hard handoff performance during the hard handoff tests is determined to be unsatisfactory. A final hard handoff confirmation test is then performed in the hard handoff area following the latter communication equipment adjustments.
The adjustments to the communication equipment to improve the hard handoff success rate may include: adjustments to the base station RF transmit power in the control channel or pilot channel, adjustments to the base station antenna beam in azimuth and/or elevation (tilt), and adjustments to base station software.